1. Field of the Invention
This invention relates to the field of high-quantum efficiency detectors, and is of particular use in the field of detectors with a thin absorption layer.
2. Description of Related Art
As is known per se, a photodetector comprises a semiconductor layer capable of absorbing photons in order to convert them into electron hole pairs which are collected to generate an electric current.
The quantum efficiency of a photodetector, which is defined as the ratio between the number of photons received by the photodetector and the number of photons absorbed thereby, is therefore the principal characteristic determining the quality of the photodetector. Since said efficiency is related to the absorption capacity of the semiconductor layer, it is easily understood that a large volume of semiconductor material allows high levels of efficiency to be attained.
However, a semiconductor layer of reduced volume also offers a certain number of advantages. Photodetectors with a semiconductor layer of reduced volume thus offer a reduced material cost, are faster, or have an improved signal-to-noise ratio as regards generation-recombination noise.
However, the drop in quantum efficiency caused by reducing the absorption volume is also accompanied by other disadvantages.
In particular, as the thickness of the semiconductor absorption layer diminishes, it lets more and more photons through without absorbing them, and therefore becomes increasingly “transparent” to the radiation for detection.
To enhance light absorption in thin semiconductor layers deposited on growth substrates, solutions are thus usually considered for placement on the rear surface of the semiconductor layer, i.e., the surface opposite the one receiving the incident radiation, so that at least some of the radiation that has passed through the semiconductor layer without being absorbed can be “recovered”.
A first solution comprises placing a plane reflector, for example a metal mirror or a Bragg mirror, on the rear surface of the semiconductor layer so as to reflect the non-absorbed light back towards it. This solution enhances absorption by sending the light through the semiconductor layer twice without particular resonance. This solution may however prove inadequate in the case of layers that are very thin or too unabsorbent, in other words if the double-thickness passed through still does not allow full absorption.
In the situation where the plane mirror does not suffice, another solution comprises using a textured rear reflector, which serves to optimize quantum efficiency enhancement by directing the radiation more along the absorption layer. Indeed, texturing allows the radiation for detection to be coupled with a trapped mode of the semiconductor layer. This solution is clearly more effective than using a straightforward plane mirror. Furthermore, it uses a diffraction phenomenon through a grating which makes the detection sensitive to the wavelength of the incident light via the grating period. This applies for example to solar cells, as shown in the reference work “Optical Properties of Thin-film Silicon Solar Cells with Grating Couplers” by C. Haase and H. Stiebig, Progress in photovoltaics: research and applications, vol. 14, p 629-641 (2006), with a one-dimensional diffraction grating of the type with silver grooves, having a typical thickness of λ/4nSi.
As such, it is remarkable to note that texturing serves to a certain extent to transform the disadvantage of the “transparency” of the semiconductor absorption layer into an advantage. Thus, since the quantum efficiency gain obtained by coupling to the trapped mode of the semiconductor layer is by no means insignificant, the thickness of the semiconductor layer is intentionally selected as “thin” in order to let some of the radiation through so that significant coupling can be obtained through the texturing.
“Thin” semiconductor layer is taken in terms of the invention to mean a semiconductor layer whereof the thickness is selected in such a way that some of the radiation of interest passes through said layer without being absorbed. For example, a semiconductor layer whereof the thickness t verifies
      t    ≤          λ              3        ×                  Im          ⁡                      (                          n              1                        )                                ,where λ is the wavelength for detection, n1 is the refractive index of the semiconductor layer and Im in denotes the imaginary component, is considered as thin in terms of the invention since it lets through the wavelength for detection. This equates in particular to a thickness t of less than one micrometer in the infrared spectrum.
Furthermore, as may be observed, the structuring thickness depends on the wavelength for detection. Thus when this type of structuring is used in the infrared, it is necessary to implement structures of substantial thickness. In fact, it is difficult to structure such thicknesses in a metal material given, for example, cap lift-off problems, problems of filling deep cavities, problems of controlling the depth of a deep etch in a dielectric when the thickness between the bottom of the cavity and the absorbent semiconductor layer needs to be accurately controlled, problems of etching noble metals such as gold which require high-temperature plasma-based methods, and typically temperatures above 200° C. For example, the document WO2005/081782 discloses a detector that combines a textured rear reflector with a semiconductor absorption layer. In this document, it is proposed to use a waffle-type coupling grating, placed to the rear of an absorption layer formed from a stack of quantum wells so as to increase absorption in a range of infrared radiation. The purpose of the periodic grating with square holes is in this case to reflect the radiation passing through the absorption layer without being absorbed while dispersing it therein. Enhanced quantum efficiency is observed, but this enhancement is obtained by combining the quantum wells and the reflector and additionally requires the reflector to be structured with a thickness of about one quarter of the wavelength, which is highly significant when working in the infrared spectrum.
It can be seen as a result that for textured rear reflectors of the prior art, the structuring thickness is strongly coupled to the wavelength for detection, which offers a certain number of disadvantages, particularly in the infrared spectrum.